In telecommunication, an error vector magnitude (EVM) represents the difference between a measured carrier magnitude and phase at a point in time, and the expected magnitude and phase at that same point in time. EVM is typically measured to quantify the amount of degradation or distortion in a digital signal. The effect of EVM degradation can be attributed to a variety of reasons such as phase noise, compression point, IQ balance, T/R pulling distortion, AM variations, and FM variations, some of which have a dependency on the following external time-changing factors such as temperature, supply voltage, load pulling by an antenna, and antenna proximity to materials that affect its impedance. For wireless transmissions at relatively high data rates (e.g., up to 54 Mbps under the IEEE 802.11g standard), high output power coming from the transmitter tends to compress the power amplifier output and degrade the EVM enough to compromise the link quality. At these high data rates, increasing transmitter power in a properly designed compression point limited system usually decreases the range of effective transmission because of the increased EVM degradation caused by over-driving the power amplifier.
Many solutions have been proposed to control the effects of EVM degradation caused by antenna load variation. These solutions are typically hardware based, potentially expensive, and may not eliminate the problem but only reduce the impact. For example, the most obvious solution to antenna load shifting is to incorporate an isolator between the power amplifier and the antenna. Apart from causing insertion loss, this solution is too cumbersome and too expensive for a wireless local area network (WLAN) radio. To account for the effects of varying voltage and/or temperature, some hardware designers include sensors and calibration in the transmitter. The most common sensor is a voltage detector. However, the antenna load tends to shift due to its proximity to external materials. The antenna load shifts impact the power amplifier's compression point causing EVM distortion and therefore degrading the link quality. The antenna load shifts also degrade the system performance by adjusting the voltage to power calibration by changing the power amplifier load line. The result is an inability for the voltage detector to accurately control the output power in relation to the power amplifier's compression point. This uncertainty requires an excessive power reduction from the normal operational point to account for these errors. The IEEE 802.11 Task Group k is currently looking into techniques that attempt to improve link performance, but those techniques require feedback from the receiver to adjust the transmitter. In addition, those techniques are developed to minimize interference to neighboring networks, not to directly improve the transmitter performance or link quality.
In a transmitter design, the power amplifier compression typically dominates the signal distortion. Thus it would be a good practice to balance the power amplifier distortion with the ability of a receiver to cope with that distortion. Most standards bodies adopt a minimum quality metric for the transmitted signal. For the IEEE 802.11 case, the chosen metric is EVM. A transmitter is manufactured to operate above a minimum EVM level over all operating conditions including wide ranges of temperature, supply voltage and antenna load. Such configuration mandates a removal of the signal away from the power amplifier's compression point by reducing the output power by a margin to account for these external factors. Unfortunately, this margin may significantly impact the total output power of the transmitter, reducing its overall range for nominal operating conditions.
In view of the foregoing, it would be desirable to provide an efficient and cost effective solution for counteracting EVM degradation that overcomes the above-described inadequacies and shortcomings.